A standard hammer drill has a drive spindle that is rotated about a spindle axis and a hammer displaceable axially in the drive spindle. A chuck is carried on the spindle so as to rotate therewith, and holds a drill bit having a shaft having a radially outwardly open and at least generally axially extending groove. The chuck holds the drill bit so it is centered on the spindle axis, and the hammer is effective on the rear end of the drill bit. The bit is rotationally coupled to the chuck body, which as mentioned above is rotated, and is at least limitedly axially reciprocated by the hammer so that it can drill through masonry or the like.
A light-duty hammer drill normally has a simple Jacobs chuck comprising a chuck body comprised of one part formed with a frustoconical surface centered on the chuck axis and another part axially displaceable relative thereto and forming an abutment surface generally perpendicular to the chuck axis. Three angularly equispaced jaws are axially braced between the centering and abutment surfaces and are urged radially apart by springs. The two parts are threaded together so the surfaces can be axially relatively displaced, thereby radially displacing the jaws. To hold a bit, the parts are relatively rotated to separate the surfaces and spread the jaws, the shank of the bit is placed between the spread jaws, and the parts are oppositely relatively rotated to clamp the jaws on the shank of the bit.
Such an arrangement axially couples the bit to the chuck so that the hammering is not highly effective. Accordingly German patent document 3,133,085 filed Aug. 21, 1981 by the instant applicant proposes a chuck where the jaws that rotationally couple the bit and chuck body are mounted in a core that can move limitedly axially on the chuck body, while of course being rotationally coupled thereto. The coupling jaws of such an arrangement can bite into and imbed themselves in the cylindrical drill shaank, damaging the bit and making it difficult to remove from the chuck. If they bite in, their grip actually will loosen, so that the bit will slip and be damaged. Once the shank of a bit is damaged in this manner, it becomes difficult or impossible to chuck it on center.
German utility model 8,132,988 filed Nov. 11, 1981 by Hilti describes a drill bit formed with axially elongated and radially outwardly open grooves. Special formations on the chuck can engage in them, while permitting relative axial movement, to transmit torque from the chuck body to the drill bit. This type of chuck cannot, however, be used with a conventional bit, only with special grooved ones, a distinct disadvantage since most hammer drills allow the hammering to be canceled for use as a standard purely rotary drill.